Problem adding some user or active directory group to sharepoint 2010 group

Hi All

I have a problem in a specific site collection in a web Application (but not on other site collection in that webApp).

whenever I add a user like some system account to a sharepoint group or create a new sharepoint group or add an ActiveDirectory group to a sharepoint group I get an error and the user / group are not added :

System.Runtime.InteropServices.COMException: [Work Email Address] - [Wrong Email Format]    at Microsoft.SharePoint.Library.SPRequestInternalClass.EnsureUserExists(String bstrUrl, String bstrLogin, String bstrEmail, String bstrName, String bstrNotes, String bstrMobilePhone, Int32 lFlags, Boolean bIsRole, Boolean bSendEmail, Boolean bForceAdd, Byte[]& ppsaSystemId, Boolean bImportDeleted, Int32& plUserId)     at Microsoft.SharePoint.Library.SPRequest.EnsureUserExists(String bstrUrl, String bstrLogin, String bstrEmail, String bstrName, String bstrNotes, String bstrMobilePhone, Int32 lFlags, Boolean bIsRole, Boolean bSendEmail, Boolean bForceAdd, Byte[]& ppsaSystemId, Boolean bImportDeleted, Int32& plUserId)

when I add a regular user - all goes well.

10x for any help

Shlomy